GRAS overview

The GRAS instrument will receive high quality radio signals from GPS navigation satellites, occulting the Earth atmospheric limb, through a tangential path through the Earth's atmosphere.

GRAS can track up to eight satellites for navigation purposes, two additional satellites for rise and two others for set occultation measurements. GRAS has on-board GPS satellite prediction for optimising the navigation and occultation measurements.

The Doppler shift in the received signals can be processed to obtain vertical profiles (at least from 1 km to 30 km) of atmospheric parameters such as temperature and pressure with a high degree of accuracy. In the stratosphere and upper troposphere, where water vapour density is low, refraction is dominated by the vertical temperature gradients, and an accurate temperature profile can be retrieved. In the lower troposphere, the water vapour effects are dominant and the combined temperature/water vapour profile can be retrieved. Measurement data from GRAS will be combined with data received in GPS ground based receivers (providing Precise Orbit Determination) to retrieve the final atmospheric profiles.